Similar experience here. Daydreaming happens more often as I spend extended periods alone, sometimes to a point I would also call "maladaptive". I'm looking for ways to deal with that. It often prevents me to make any meaningful advance on hobbies or any other activity for whose outcome there's nobody waiting -- hence, I have a number of unfinished stuff laying around and getting dust.

I agree with your thesis of under-stimulation. I wouldn't say I'm too reclusive, but social activities in general have been not particularly engaging, sometimes even a bit tiring.